Buoyancy control systems as used in SCUBA diving generally employ a collapsible bag carried by the diver and into which air is supplied to inflate the bag and increase the buoyancy of the diver. A major disadvantage of these systems is that the buoyancy compensation provided by the bag changes as the depth thereof changes unless air is supplied to or removed from the bag. For example, as the diver descends, the bag contracts under the increased ambient pressure and thus displaces less water and provides less buoyancy compensation. The reverse is true as the diver ascends. Consequently, when using such prior art systems the diver is frequently required to supply air to or release air from the bag in order to maintain his buoyancy relatively constant.
In U.S. Pat. No. 3,161,028 there is described a buoyancy control system using a constant volume tank into which water is admitted or expelled to adjust the buoyancy thereof. In the system described in the said patent the pressure in the tank is maintained within 2 p.s.i. of the ambient by means of a pair of check valves. The one of the two check valves which opens when the tank pressure falls more than 2 p.s.i. below ambient pressure is connected to the line between the conventional demand regulator and the mouthpiece through which the diver normally breathes while under water. An inherent danger in using this system is that should the diver descend with the mouthpiece in his mouth, the substantial pressure differential between the ambient and his internal cavities connected to the mouthpiece is extremely dangerous. Other problems associated with the said patented system are one, he cannot increase his buoyancy while he is in an inverted position, two, the time required to initially fill the tank with water is relatively long, and three, should the diver exhaust his main air supply, he cannot use the system to increase his buoyancy by any substantial amount to enable him to get to the surface as soon as possible.